This invention relates generally to gas turbine engines and in particular to a joint assembly for coupling a ceramic member to a metal member.
It has long been recognized that the efficiency and performance of gas turbine engines could be improved by increasing the temperature of the gas through the turbine section. Historically, these temperatures have been limited by the materials, usually high temperature steel or nickel alloy, used in forming the turbine rotor. To permit higher gas temperatures it has been proposed to form the turbine rotor from a high density, high strength, silicon nitride, or silicon carbide ceramic which can withstand higher temperatures than steels or nickel alloys.
The ceramic turbine rotor extracts energy from the hot gas in the engine and converts it into torque. To transmit this torque to other components of the engine and thereby cause them to rotate, the ceramic turbine rotor needs to be coupled to a superalloy or metal component of the engine such as the compressor rotor. Because monolithic ceramic has a higher compressive strength in comparison to its tensile strength, it is advantageous to use an interference fit type joint to couple the turbine rotor to the compressor rotor. Typically, this joint is comprised of a metal sleeve pressed on to a shaft portion of the ceramic turbine rotor with the metal sleeve applying a compressive load to the ceramic shaft portion. A disadvantage to this type of joint is that due to the thermal expansion mismatch between the metal and ceramic, a conventional interference fit loses its torque transmitting capability at temperatures in the range of 400 to 500xc2x0C. depending on the specific metal and ceramic chosen and the magnitude of the interference fit.
Accordingly, there is a need for a joint assembly for coupling a ceramic member to a metal member that is capable of transmitting torque at temperatures greater than 500xc2x0 C.
An object of the present invention is to provide a joint assembly for coupling a ceramic member to a metal member that is capable of transmitting torque at temperatures greater than 500xc2x0 C.
The present invention achieves this objective by providing a joint assembly for coupling a ceramic member to a metal member that is comprised of a ceramic shaft portion attached to the ceramic member; a slotted shrink fitter formed of a first metal and disposed around and in torque transmitting contact with the ceramic shaft portion to define a first interface surface therebetween; and a sleeve formed of a second metal disposed around and in torque transmitting contact with the shrink fitter to define a second interface surface therebetween. Either the shrink fitter or the sleeve is attached to the metal member. The coefficient of thermal expansion of the second metal is relatively low compared to that of the first metal and the diameters of the first interface, (d1), and the second interface, (d2), are determined from the equation, d1!d2=(xcex1cxe2x88x92xcex11)/(xcex12xe2x88x92xcex11) where xcex1c, xcex11 and xcex12 are the coefficients of thermal expansion of the ceramic, the first metal and the second metal, respectively.
A solid lubricant is introduced at the interface between the shrink fitter and the ceramic member to provide a shear limiting layer to prevent cracking of the ceramic member due to the axial thermal expansion mismatch between the ceramic member and the shrink fitter. Without this solid lubricant, and depending on the coefficient of friction at this interface, the magnitude of the thermal mismatch, the magnitude of the interference fit, and the service temperature, very high shear stresses can be transmitted as tensile stresses to the interface of the ceramic member once the joint assembly is heating up to its operating temperature, which will ultimately result in cracking of the ceramic. The solid lubricant will allow the ceramic to slide axially with respect to the shrink fitter once the sheer stress exceed the yield strength of the solid lubricant, thereby limiting the shear stresses to an acceptable order of magnitude for the ceramic.
By selecting the appropriate metals and interface diameters, the contact pressure, which is directly related to torque transmitting capability, between the ceramic to metal interface will remain essentially constant even at temperature above 500xc2x0 C.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.